High strength polyester filamentary yarn

ABSTRACT

A polyester multi-filamentary yarn having a novel internal microstructure, polyester tire cord produced from the yarn and a process for production thereof are provided. The polyester yarn has a novel three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions which are changed to a structure having crystalline and amorphous portions during formation of a tire cord. The polyester resin contains at least 90 mol. % polyethylene terephthalate. The resin is melt-spun and solidified by quenching to produce an undrawn yarn having a birefringence of 0.03 to 0.08, which is then drawn at a total draw ratio of 1.4:1 to 2.2:1 and thermally treated and relaxed. The resulting filamentary yarn is dipped in a rubber solution to produce a tire cord which exhibits excellent dimensional stability and fatigue resistance when it is incorporated into the rubber matrix of a tire.

This application is a Continuation of application Ser. No. 07/989,366,filed on Dec. 11, 1992, abandoned.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an industrial polyester filamentaryyarn and a tire cord formed from this filamentary yarn. Moreparticularly, it relates to a polyester filamentary yarn, a tire cordhaving improved fatigue resistance due to increased thermal dimensionalstability and strength, as well as a process for production of thisyarn.

(2) Description of the Prior Art

In general, nylon, rayon, polyester etc. fibers are typical examples ofthe fibers which have been used as reinforcements in rubber tires.

Nylon tire cord has higher strength and toughness than the othermaterials due to the inherent properties of nylon fiber and has beengenerally used in bias tires for trucks, buses etc. Rayon tire cordprovides a low degree of shrinkage and has good thermal and dimensionalstability due to the inherent properties of rayon fiber and has beengenerally used in high speed radial travelling ires for passenger cars.

Unfortunately, nylon tire cord has poor dimensional stability due to itslow modulus characteristics and high shrinkage and further exhibits flatspots due to its low glass transition temperature. Rayon tire cord alsohas low modulus characteristics and exhibits a sharp decrease instrength after the fibers have been formed into a tire cord.

In view of these defects found in both nylon and rayon tire cords,polyester tire cord has been widely used.

Prior art polyester fibers that have been used in tires have benzenerings in their molecular structure, and a rigid molecular chain.Accordingly, tire cord formed from polyester yarn has a good elasticmodulus, good fatigue resistance, provides few flat spots, excellentcreep resistance and excellent endurance. For these reasons, polyestertire cord has been widely used in radial tires for passenger cars.

However, in spite of the above described merits, conventional polyestertire cords do have a problem: they undergo substantial variation intheir properties with temperature due, it is thought, to hysteresiseffect. In particular conventional industrial high strength polyesterfibers generally exhibit substantial shrinkage when heated.

Also, when industrial polyester fibers have been incorporated into arubber matrix of a tire, as the tires rotate during use the fiber isstretched and relaxed during each tire rotation. Further, the internaltire air pressure stresses the fiber, and tire rotation while axiallyloaded or stressed causes repeated stress variations, particularly onunsmooth surfaces.

Since more energy is consumed during the stretching of a fiber than isrecovered during its relaxation, the difference of energy dissipates asheat. This is termed hysteresis or work loss. Significant temperatureincreases have been observed in rotating tires during use which areattributable at least in part to this fiber hysteresis effect.

The variation in properties caused by heat generation occurs due tomoisture and amines contained in conventional rubber solutions used inrubber treatments for producing tire cord, and the observed variationtends to be increased when the content of carboxyl group is high,leading to a significant lowering of strength and fatigue resistance.

In recent years, as radial tires having high performance have beenwidely developed and used, the demand for polyester tire cords withsuperior properties, especially properties superior to those obtainedwith nylon or rayon tire cord, has been increasing. Therefore, researchinto development of a polyester tire cord having improved fatigueresistance by minimizing the heat generated due to the hysteresis effecthas been undertaken.

Prior art methods for improving fatigue resistance of polyester fibershave focused on a chemical method for increasing stability by reducingthe content of carboxyl groups in the polyester and a method whereinhighly-oriented undrawn yarn produced using a polyester with arelatively low I.V. (intrinsic viscosity), or produced by employing ahigh-speed spinning process, is drawn.

References directed towards increased chemical stability are Japanesepatent laid-open No. Sho. 54-132696 and 54-132697 which disclose theinhibition of deterioration due to thermal decomposition resulting fromheat generation by reducing the carboxyl group content of the polyester.By reducing the content of terminal carboxyl groups via copolymerizationwith or melt-blending in an aliphatic polyester. The increased mobilityof amorphous regions effects a reduction in heat generation leading to areduction in thermal decomposition which effects improved fatigueresistance. But in this method, high crystalline polyester fiber cannotbe obtained and the tenacity and initial elastic modulus of the materialis always low. Thus, the shrinkage of the resulting fiber is increasedand the product obtained is not a high quality tire cord yarn. Also,reducing the content of terminal group by adding a blocking agent has adisadvantage in that the degree of polymerization is lowered and thecost is increased.

References directed to a method of increasing thermal stability by highspeed drawing are U.S. Pat. No. 4,101,525 and 4,195,052 which disclosean improvement in fatigue resistance by increasing the mobility of themolecular chains in the amorphous region high-speed spinning. In thisprocess, the fatigue resistance is improved but the molecular chainlength is irregular and long, and the relaxed molecular chains coexistso that the loss of tenacity is high. Also, a difference in propertiesbetween the inner and outer layer of fiber is effected so that thedrawability deceases. The resultant variation in physical propertieswithin these regions of the fiber is severe due to the presence of adefective microstructure.

Prior art processes for producing tire cord from yarn include, forexample, Japanese patent laid-open No. Sho 61-12952 which discloses atire cord having a tenacity of at least 7.0 g/d, an absorption peaktemperature in the amorphous region of 148°-154° C., a shrinkage of3.3-5% by spinning a polyester polymer having an intrinsic viscosity of1.0, a diethylene glycol content of 1.0 mol %, a carboxyl group contentof 10 eq/10⁶ g at a spinning speed of 2,000-2,500 m/min to obtainundrawn yarn, drawing the undrawn yarn at about 160° C. thermallytreating at 210°-240° C. and dipping the obtained yarn in a conventionalrubber solution.

In addition, U.S. Pat. No. 4,101,525 and 4,195,052 disclose a polyestertire cord produced by a process comprising drawing highly orientedundrawn yarn prepared from a high-speed spinning process to obtainhighly oriented drawn yarn, specifically multi-drawn yarn comprising 85mol % polyethylene terephthalate having a denier per filament of 1 to 20and a work loss at 150° C. of 0.004-0.02, and dipping the multi-drawnyarn in a rubber solution.

In the above methods, tie molecules, which have an important effect upondimensional stability (especially shrinkage) are oriented. This leads toresidual internal stress and finally causes a lowering of the fatigueresistance of the tire cord. In most of the conventional polyester yarnsfor tire cord, internal stress produces a temperature rise which inducesa continuous increase of thermal stress. This finally results in poortire cord fatigue resistance because after the tire cord conversionprocess (or dipping process) comprising dipping the cord in a rubbersolution and thermally treating, an internal stress of about 0.5 g/dusually remains in the tire cord.

Moreover, yarns which are highly oriented drawn yarns before undergoingthe tire cord conversion process have a definite two-phase structurewith both crystalline and amorphous regions. When it is dipped in arubber solution and thermally treated, deterioration of the crystallineregions occurs and leads to a lowering of strength.

In addition, Japanese patent laid-open No. Sho. 61-146876 discloses aprocess for producing a polyester tire cord by spinning yarn with asmall mass flow rate per spinneret capillary to attain a relatively highSpin-Draw Ratio, thereby obtaining a highly oriented undrawn yarn at arelatively low spinning rate, producing a high strength yarn followed bydipping it in a rubber solution and thermally treating it at atemperature higher than 220° C. This process has a disadvantage in thatthe beneficial properties of the polyester yarn are lost in the twistingand dipping process due to deterioration of the crystalline portions ofthe yarn by heat, and the final dipped cord has rather poor properties.

Japanese Patent laid-open No. Sho. 54-77794 discloses a process whichcomprises treating polyester drawn yarn with an epoxy resin compoundprior to dipping in a rubber solution but this process did not solve theabove-described problems.

The present invention has been developed to solve the above describedproblems of the prior art. According to the present invention, the twoproblems of i) lowering of fatigue resistance due to residual internalstress by high-speed spinning and ii) lowering of strength due todeterioration of the crystalline portions on dipping in a rubbersolution can be solved based upon the points described below.

Polyester yarn having a high crystallinity undergoes a high degree ofthermal hysteresis and, accordingly, has a high thermal stress. Thus, ittends to undergo a lowering of strength, elastic modulus or strengthconversion efficiency due to formation of folded crystals and inparticular, from unconstrained molecular chains in the amorphous regionspresent during recrystallization which subsequent heat treatments, suchas dipping process etc., cause. In addition, though a high crystallinepolyester yarn itself may exhibit good thermal stability, dimensionalstability and fatigue resistance, the yarn has a definite two phasemicrostructure which may effect a rapid growth of crystal size or longperiod growth upon subsequent heat treatment so that the fatigueresistance initially exhibited by the yarn itself can not be obtainedafter it is twisted and subsequently heat-treated.

Conventional tire cords which have been used reinforcers in tireproduction exhibit a shrinkage of at least 10% when subjected to a hightemperature. Moreover, when they have been incorporated into the rubbermatrix of a tire, the repeated fatigue movements such as stretching,compression and flexing lower the inherent properties of the fiber suchas strength, elastic modulus and toughness. Furthermore, the poorfatigue resistance results in bad tire uniformity.

The present inventors have directed their research towards improving theprior art methods of producing a polyester yarn for tire cord which hasexcellent overall physical properties like strength, and, at the sametime, which has a high strength conversion efficiency and excellentdimensional stability lusting to excellent fatigue resistance when usedeven after having been subjected to a cord conversion process and thenincorporated into a rubber matrix. As a result of this research, thepresent invention has been achieved. Thus, whereas most of the prior artmethods comprise producing a polyester yarn having a stable two-phasestructure of crystalline and amorphous portions or regions and thensimply dipping it into a rubber solution to obtain a final tire cord,the present invention comprises producing a polyester yarn having athree-phase structure of crystalline, amorphous and mesomorphousportions and thereafter subjecting it to recrystallization during adipping process to obtain a tire cord having a stable two-phasestructure.

As the mesomorphous portions present in the yarn are crystallized whilebeing subjected to heat in a dipping process, crystals with a 10%smaller size than the crystals obtained in prior art methods areproduced and the present yarn in cord form provides dimensionalstability by developing a network structure with uniformly formedcrystalline and amorphous portions, and has a high elastic modulus dueto the minimization of the formation of folded crystals duringrecrystallization thereby increasing the content of the strained tiemolecular chains which interlink crystals.

Moreover, the present inventors have discovered particular spinning anddrawing processes which achieve the above characteristics. Consequently,the process conditions necessary to produce an excellent polyesterfilamentary yarn have been designed. In more detail, a undrawn yarn isproduced which has highly oriented molecular chains in amorphous statesuch that crystalline diffraction by x-ray is not observed, thereafterthe undrawn yarn is drawn at a low draw ratio and a low temperature(below the crystallization temperature) so as to minimize the strain ofmolecular chains in amorphous regions induced by drawing, and thensubjected to thermal treatment and relaxing at a low temperature so thatno further crystallization proceeds. The filamentary yarn is then dippedinto a rubber solution and thermally treated at certain temperature andtension conditions enabling recrystallization to occur, therebyobtaining a final polyester tire cord.

OBJECTS OF THE INVENTION

The first object of the present invention is to provide a polyesterfilamentary yarn which exhibits excellent fatigue resistance anddimensional stability both before and after it has been incorporated ina rubber matrix even under the conditions where it is subjected torepeated fatigue behavior at high temperatures (at least 210° C.), and aprocess for production thereof.

The second object of the present invention is to provide a tire cordcomprising a polyester filamentary yarn exhibiting excellent dimensionalstability and fatigue resistance useful as a reinforcement in rubber.

The third object of the present invention is to provide a tireexhibiting significantly improved fatigue resistance and dimensionalstability even under the conditions of repeated fatigue behavior at hightemperatures.

To achieve the above objects, the present invention provides a polyesterfilamentary yarn comprising at least 90 mol % polyethylene terephthalateand having a fineness of 3 to 5 denier per filament, and possessing athree-phase microstructure consisting of crystalline, amorphous andmesomorphous portions, characterized in that the proportion of saidmesomorphous portion is 5 to 15 percent base upon the total amount ofcrystalline, mesomorphous and amorphous portions.

The present invention additionally provides a polyester filamentary yarncomprising at least 90 mol % polyethylene terephthalate and having afineness of 3 to 5 denier per filament, characterized in that said yarnsatisfies the following characteristics

i) a crystalline orientation function (fc) of at most 0.94,

ii) an amorphous orientation function (fa) of at least 0.60,

iii) fa X (1-Xc)>0,330 (where, Xc is the percent crystallinity,0.30-0.45); and

iv) a long period value of at most 155 Å.

The present invention additionally provides a polyester filamentary yarncomprising at least 90 mol % polyethylene terephthalate and having afineness of 3 to 5 denier per filament, characterized in that said yarnsatisfies the following characteristics:

i) a crystallinity of 30-45 percent by weight;

ii) a crystallite face size [(105) plane] of at most 65 Å; and

iii) a crystal volume of 0.5×10⁵ Å³ -1.54×10⁵ .di-elect cons.³.

The present invention additionally provides a polyester filamentary yarncomprising at least 90 mol % polyethylene terephthalate and having afineness of 3 to 5 denier per filament, characterized in that said yarnhas a maximum thermal stress of at most 0.5 g/d in the temperature rangeof 60° to 250° C. and exhibits a decrease of thermal stress beyond 210°C.

Also, the present invention provides a process for producing a polyesterfilamentary yarn from a polyester resin comprising at least 90 mol %polyethylene terephthalate and having an intrinsic viscosity of at least0.85 by melt-spinning, drawing, thermally treating and relaxing,characterized in that:

1) said polyester resin is spun at a spinning speed of 2,500-4,000 m/minand then solidified by quenching at a quench air temperature of 25°C.-Tg of the polymer to produce a undrawn yarn;

2) said undrawn yarn is drawn at a drawing temperature of Tg of thepolymer-120° C. and a total draw ratio of 1.4:1-2.2.1;

3) the obtained drawn yarn is thermally treated at a temperature of100°-210° C.; and

4) the thermally treated yarn is relaxed at a temperature of at most140° C. and a relax ratio of 3 to 6%.

In addition, the present invention provides a tire cord formed from apolyester filamentary yarn comprising at least 90 mol % polyethyleneterephthalate, characterized in that said cord satisfies the followingcharacteristics:

i) a strength at 10% elongation of at least 100 Newtons,

ii) a shrinkage, S, of at most 3.5% obtained upon dry heat treatment at177° C. during 2 minutes under a dead weight loading of 20 g,

iii) a strength at 10% elongation after the treatment as described inthe ii) above, L, of at least 65 Newtons, and

iv) a coefficient of dimensional stability, L/S, of at least 20.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diffractogram obtained from the X-ray diffractionpatterns for a polyester filamentary yarn of the present invention.

FIG. 2 illustrates a diffractogram obtained from the X-ray diffractionpatterns for a polyester filamentary yarn of the prior art.

FIG. 3 illustrates a graph plotting the thermal stress values ofpolyester filamentary yarns vs. temperature.

FIG. 4 illustrates a graph plotting the thermal stress values ofpolyester tire cords vs. temperature.

.Iadd.FIG. 5 illustrates the polyester filamentary yarn 1 of the presentinvention having amorphous portions 2, mesophase portion 3 andcrystalline portions 4.Iaddend..

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polyester filamentary yarn of the present invention contains atleast 90 mol % polyethylene terephthalate, desirably at least 95 mol %polyethylene terephthalate with intermediate effective amounts including91, 92, 93, 94, 96, 97, 98, 99 and 100 mol %.

Additionally, the polyester yarn of the present invention may contain atmost 10 mol %, desirably at most 5 mol %, copolymerized ester unitsother than polyethylene terephthalate with intermediate effectiveamounts including 1, 2, 3, 4, 6, 7, 8 and 9 mol %.

The ester-forming ingredients useful as ester units other thanpolyethylene terephthalate include, for example, glycols such asdiethylene glycol, trimethylene glycol, tetramethylene glycol,hexamethylene glycol and dicarboxylic acids such as isophthalic acid,hexahydroterephthalic acid, adipic acid, sebacic acid, azelaic acid,etc.

The polyester filamentary yarn of the present invention usually has afineness of 3-5 denier per filament, but this value can be widely variedas is apparent to the skilled in this art, and includes yarns with afineness of from, for example, 0.1-50 denier per filament and beyond.

Polyester filamentary yarn of the present invention comprisescrystalline, amorphous and mesomorphous portions. More particularly, itis a polyester yarn having a three-phase structure wherein themesomorphous portions are present in a significantly higher amount thanin the prior art yarns having nearly a completely two-phase structure ofcrystalline and amorphous portions (wherein the mesomorphous portionsare present only in the extremely small amounts if at all).

In the present invention, the word "mesomorphous phase" or "mesophase"is defined to mean the oriented (ordered) amorphous portion of the yarnor fiber which is strictly the portion having some degree of orientationamong amorphous portions. The percentage of mesomorphous portion can becalculated from the following formula using X-ray intensity (I)separation procedures on the peak of (010) crystal plane in adiffractogram obtained by quantitatively computing the equatorial X-raydiffraction patterns. (See FIGS. 1 and 2) ##EQU1##

Polyester yarn of the present invention has 5-15% of a mesomorphousportion as calculated according to the above formula. When this value ofless than 5% the smallest amount of mesomorphous portion exists, thatis, the structure approaches the two-phase structure of the prior artyarns where I meso=0 but still provides significant benefits thereover.

On the contrary, when the mesophase portion is more than 15%,difficulties in yarn manufacturing process occur and it is difficult toobtain a sufficient tenacity as is required for a tire cord yarn. Otherparticular effective amounts of the mesomorphous portion in the presentyarn include 6, 7, 8, 9, 10, 11, 12, 13 and 14%.

When a polyester yarn of the present invention having a mesophaseportion as described above has been dipped in a rubber solution andthermally treated, the mesophase is transformed into a crystallinephase. A network structure of uniformly distributed crystalline andamorphous portions thus develops providing high dimensional stability.Further, the lowering in strength due to recrystallization is remarkablydecreased.

In addition, as the mesophase becomes, or is incorporated into, acrystalline phase, the number of tie molecular chains in the amorphousportion is increased. The tie molecular chains yield good moduluscharacteristics.

The polyester filamentary yarn of the present invention is characterizedby a crystallinity of 30-45 percent by weight.

When it is more than 45 percent by weight, the yarn has a definitestructure of crystalline and amorphous portions which yieldssignificantly lower strength as it is subjected to high heat, and theproperties of the finally obtained dipped cord are poor. When thecrystallinity is less than 30 percent by weight, the filament is so softthat breakage frequently occurs during the yarn manufacturing process.Other effective crystallinities include 32, 34, 36, 38, 40, 42 and 44percent by weight.

Therefore, according to the present invention any lowering in strengthof the yarn is minimized by producing a yarn having a three-phasestructure, wherein the crystalline and amorphous phases are not clearlydivided due to the coexistence of a mesomorphous phase, allowing onlythe appropriate stress isolation, and then forming a completed structureof crystalline and amorphous portions by a cord conversion processcomprising dipping the yarn in a rubber soln.

The crystallinity may be determined from the following equation usingthe density (ρ, unit: g/cm³) of the fiber. ##EQU2## where, ρc(g/cm³)=1.445

ρa (g/cm³)=1.335

The density (ρ) may be determined by measurements according to densitygradient column method using n-heptane and carbon tetrachloride at 25°C.

Furthermore, polyester filamentary yarn of the present invention ischaracterized in that the value of fa(1-Xc) is equal to or more than0.33. This expression is intended to define a particular characteristicof the amorphous portion of the material for the purpose of obtainingthe three-phase structure of the present invention.

When fa(1-Xc) is less than 0.33 it means that the yarn crystallinity ishigh or the proportion of oriented amorphous portion is low, which as isfound for products like conventional yarns having a two-phase structure.High crystallinity causes a lowering of strength by the action of heatwhen the yarn is treated to form a tie cord. Alternatively, when theproportion of oriented amorphous portion is low, it becomes difficult toobtain the strength necessary for a tire cord. For this reason, it isnecessary to control the characterization parameters of the yarn towithin the above preferable range: an amorphous orientation function(fa) of at least 0.6 and a (1-Xc) value of 55-70 percent by weight.

In addition, the present yarn is characterized by a crystallineorientation function (fc) of at most 0.94 and an amorphous orientationfunction (fa) of at least 0.60.

The crystalline orientation function (fc) may be calculated from thefollowing equation (1) by averaging the orientation functions obtainedfrom the width at half-height (Xh_(kl)) of the wide angle X-raydiffraction pattern of ebd (010) and (100) planes of the material:##EQU3## where ##EQU4##

The amorphous orientation function (fa) may be calculated from thefollowing equation (2). ##EQU5## where Δn_(c) =intrinsic birefringenceof crystal (0.220)

Δn.sub.α =intrinsic birefringence of amorphous (0.275) The birefringence(Δn) may be calculated from the following equation (3) by measuring theretardation obtained from the interference fringe by the sample using aBerek compensator mounted in a polarizing light microscope.

    Δn=R/d                                               (3)

where

d=thickness of sample (nm)

R=retardation (nm)

When the crystalline orientation function is more than 0.94, thecrystalline and amorphous portions are clearly defined so that bythermal treatment after dipping, a decrease in strength due to theundesirable growth of crystals and formation of folded chains on thesurface of crystal occurs, and the lowering of modulus due to the lowdegree of orientation in the amorphous portions may be effected therebyto produce a final dipped cord with poor properties. On the contrary,when the amorphous orientation function is less than 0.60, the yarn isalready in the stress-isolated state so that properties of the finaldipped cord also become poor although the dimensional stability of theyarn may be good.

In the present invention, a yarn with a crystalline orientation functionof at most 0.94 and an amorphous orientation function of at least 0.60is a yarn wherein the crystalline and amorphous portion boundaries arenot clearly defined and, besides the crystalline and amorphous portions,the pseudo-crystalline of mesomorphous portion exists.

Such pseudo-crystalline portion corresponds to the mesomorphous phase(mesophase) portion and it mostly exists between microfibrils. Thisintermicrofibrillar tie molecules are constrained by the crystallineportions present in the microfibrils and thus strongly resistant againstdeformation inflicted from the outside. This enables the attainment ofhigh modulus characteristics and a decrease in shrinkage bydisorientation of amorphous molecular chains at high temperature.

In "Now Polyester Tire Cord Development", by Richard R. Dean, appearingin Polyester Textiles, 1988, p 195-212, it is reported that in order toproduce a polyester tire cord having good dimensional stability, theeffect of intermicrofibrillar tie molecules should be maximized toprevent shrinkage and fibrillization, and fatigue resistance should beincreased. As evidenced with the goal of this article, Applicants'unique material has a microstructure which maximizes dimensionalstability.

Thus, it is necessary, in order to attain a stabilized structure in thepresent yarn, to provide a yarn with a crystalline orientation functionof at most 0.94 and an amorphous orientation function of at least 0.60.When the present yarn which has such a structure is thermally treatedafter dipping, part of the intermicrofibrillar tie molecules areincorporated in the crystalline portion and the remainder will exist asintermicrofibrillar tie molecules in the dipped cord. Thereby, yarns areobtained with high dimensional stability which avoid phenomena such asthe undesirable increase in long period length and a decease of linkingmolecular chains caused by dipping process. That is, a dipped cord withexcellent properties is obtained.

In "The Effect of Spinning Speed and Drawing Temperature on Structureand Properties of Poly (ethylene terephthalate) yarns", by R. Huiman andH. M. Heuvel, appearing in Journal of Applied Polymer Science, 1989,Vol. 37, p 575-616, it is reported that the shrinkage of polyester fiberis proportional to the product of the amount of amorphous material andthe degree of amorphous orientation.

Besides these facts, the present inventors have discovered that thefollowing fact is more important in producing a reinforcing fiber forrubbers such as tires.

A reinforcing fiber for rubbers such as that used in tires under fatiguebehavior, including repeated stretching, compression and flexing, isrequired to have improved toughness and high dimensional stabilitybecause it tends to suffer a severe lowering of strength and elasticmodulus.

In order to achieve these characteristics, the crystalline portions ofthe material should be uniformly distributed in the yarn.

The shrinkage phenomenon, which is an important indicator of dimensionalstability, is a phenomenon observed when molecular chains are subjectedto heat: the length of the molecular chains is shortened as themolecular orientation of amorphous portions becomes loose anddisordered.

The part which contributes to a decrease in the dimensional deformationis the crystals adjacent to the amorphous portions. If such crystalsform a close network structure, in other words, if the size of thecrystals is smaller at equal crystallinity and the value of long period,which is a parameter reflecting the size of the crystal and amorphousregions, is small and a close and regular macrostructure has beenformed, such a structure can act to prevent the dimensional deformationsuch as shrinkage by heat. This yields the same effect as that offorming a crosslink network in a rubber using sulfur.

Thus, enhancing dimensional stability has been limited to a certainextent without the improvement of crystallite size, crystallitedistribution and long period.

The present yarn is characterized by a close structure wherein thecrystallite size of the plane (105) crystal plane perpendicular to themolecular chains is at most 65 Å, the magnitude of crystal volume isfrom 0.5×10⁵ Å³ to 1.54×10⁵ Å³, and the long period is at most 155 Å.

The long period value may be calculated from the Bragg equation byobtaining the small angle X-ray scattering pattern under the conditionsof 50 kV voltage, 200 mA current employing X-ray scattering instrument(the inventors used an instrument manufactured by RIGAKU Co., Ltd. ofJapan), using Cu-K α radiation with 1.54 Å wavelength as a light source.

    d=λ/2θ(Bragg equation)

where

λ=1.54 Å

θ=scattering angle

The crystallite sizes [(100), (010) and (105) reflections] may becalculated by obtaining the width at half-height from the equatorial andmeridian x-ray diffraction pattern and substituting it in Scherrerequation. ##EQU6## where K=0.9

λ=1.54 Å

.[.=(b² -B²)^(1/2).]. .Iadd.β=(b² -B²)^(1/2).Iaddend.

(b: the width at half-height of peak, B: instrument constant)

The crystal volume may be calculated from the following formula

    Crystal volume=(crystal size of a axis)×(crystal size of b axis)×(long period)×(crystallinity)

Furthermore, the present filamentary yarn is characterized by a terminalmodulus of at most 20 g/d.

In general, it is known that the higher the initial modulus or terminalmodulus, the more severe the lowering of strength upon twisting anddipping process. However, the present inventors have found that suchlowering of strength is more affected by terminal modulus than byinitial modulus. But in the case where significant crystallization hasprogressed in the yarn, the lowering of strength is severe though theterminal modulus is low. Thus, by a high relaxing ratio or strong heattreatment the terminal modulus may be lowered even to a minus(-) value,but in this case the lowering of strength upon twisting and dippingcannot be avoided owing to a high crystallinity.

Another characteristic of the present yarn is thermal stress behaviouras if depends on applied temperature.

A yarn used for reinforcing a rubber is required to have excellentmechanical properties and thus strong tension in the face of drawing andthe application of high heat yields accumulated stress.

To isolate most of the stress, a relaxing process has been used in theproduction of yarn. But the inventors have found as a result of theirresearch that there is a limit in isolating the stress when employing arelaxing process. This is because the accumulated stress is mostly thestress caused by heat generated from drawing, thermal treatment etc.

Therefore, in a conventional yarn producing process the stress isolationinevitably has been limited to an extent.

In addition, another factor to limit the stress isolation present in themethods to isolate stress employing a relaxing process is that even ifthe degree of orientation is lowered to less than 0.6 as in U.S. Pat.No. 4,101,525 and U.S. Pat. No. 4,195,052, the constraint of theamorphous molecular chains cannot be sufficiently released owing to thefolded molecular chains on the crystal surface and a high amount ofdefects on the crystal interface, and it is not easy to obtain highelastic properties due to the decrease of the proportion of tiemolecules.

Therefore, the present yarn is characterized in that the maximum thermalstress is at most 0.5 g/d in the temperature range of 60° C. to 250° C.

All after-treatment temperatures employed to transform a polyester yarninto a reinforcement for rubbers usually exceed 210° C. The conventionalpolyester yarns for reinforcing a rubber, particulary a tire, normallydo not exhibit high thermal stress but do exhibit fairly high thermalstress at high temperatures greater than 210° C. On the contrary, thepresent yarn has a characteristic where the thermal stress dropsgradually beyond 210° C. and the final cord may have a thermal stressless than 0.1 g/d. Accordingly, heat-generation and dimensionalproperties can be improved significantly so that a cord with excellentfatigue resistance can be produced.

Consequently, the present yarn itself exhibits a high shrinkage of about8 to 15% in an oven at 150° C. during 30 minutes under zero tension butis a polyester filamentary yarn for reinforcing a rubber with excellentdimensional stability and fatigue resistance owing to the factors suchas the above described microstructural characteristics, the compactmacrostructure, the network structure of crystals and the significantdecrease of thermal stress at high temperatures.

The characteristics of the polyester filamentary yarn will now bedescribed by way of the thermal stress curves in FIG. 3.

FIG. 3 illustrates the thermal stress curves obtained from testing thepolyester filamentary yarns of 1,000 denier. Curve (a) represents thethermal stress behaviour of a polyester yarn for a tire cord accordingto the prior art (Comparative examples 8) and curve (b) represents thatof a prior art polyester yarn with slightly improved dimensionalstability (Comparative example 9) and curve (c) represents that of apolyester yarn of the present invention (Example 5). The testing methodused was the measurement of the thermal stress at from ambienttemperature to 265° C. with a heating rate of 2.5° C./sec under aninitial load of 50 g by a thermal stress analyzer manufactured by KANEBOCo. Japan. The figure demonstrates that the thermal stress of thepresent yarn decreases beyond 210° C.

A tire cord of the present invention produced from polyester filamentaryyarn of the present invention as described above exhibits excellentproperties as follows:

i) a strength at 10% elongation (L10) of at least 100 Newton.

ii) a shrinkage, S, of at most 3.5% obtained upon dry heat treatment at177° C. during 2 minutes under a dead weight loading of 20 g;

iii) a strength at 10% elongation after the treatment in the above ii),L, of at least 65 Newton; and

iv) a coefficient of dimensional stability (L/S) of at least 20.

In addition to the above characteristics, the present tire cord has amaximum thermal stress of at most 0.1 g/d as can be seen in FIG. 4.

FIG. 4 illustrates the thermal stress curves obtained from testing thepolyester tire cords produced by subjecting the polyester filamentaryyarns similar to those used in FIG. 3.

Curve A represents the thermal stress behaviour of a prior art polyestertire cord (Comparative example 17), curve B represents that of a priorart polyester tire cord (Comparative example 15) and curve C representsthat of a tire cord according to the present invention (Example 24). Thethermal stress was measured at ambient temperature to 300° C. with aheating rate of 2.5° C./sec under an initial load of 50 g by KANEBOthermal stress analyzer.

From the figure, it can be seen that the present tire cord exhibits asignificantly lower thermal stress compared to the tire cords of theprior art.

The dipped cord of the present invention exhibits excellent thermalstability, leading to a low shrinkage, because the network structurecomprising the uniformly distributed crystalline portions develops wellin the cord.

The present inventors have achieved a means to increase the dimensionalstability with a lowering of the shrinkage by producing a dipped cordhaving a network structure wherein the crystalline and amorphousportions are uniformly distributed.

It is generally known that a tire cord having a network structure asdescribed above generates a lot of heat, when it has been incorporatedin a tire and subjected to deformation power such as elongation andcompression, because a high activation energy is required for themolecular chains existing in the amorphous portions to move, and, as aresult, the interior temperature of tire will be increased andaccordingly the tire cord will have poor fatigue resistance and a shortlifetime. But in practice the opposite phenomenon is observed.

From "Effects of Structure on the Tensile, Creep and Fatigue Propertiesof Polyester Fibres" by A. R. Bunsell, appearing in Journal of MaterialScience, 1987, Vol. 22, p 4292-4298, it is known that fibers having anetwork structure wherein the crystals with a small size are uniformlydistributed exhibits good fatigue resistance. The present inventors alsohave verified by experiments that the above described networkcontributes to excellent fatigue resistance. This is because the fatiguemechanism of tire cord is attributed to the chemical deterioration muchmore than to physical deterioration.

From "Research for Deterioration of Polyester Tire Cord in Tire" inJournal of Japanese Rubber Associates, 1991, Vol. 64, p260-266, it isknown that about 80 percent of the deterioration due to fatigue iscaused by the hydrolysis and aminolysis of ester bonds in the polyestermolecular chains and the remainder is caused by physical deformation.

If a tire cord in tire has a structure wherein a network develops well,movement of the amorphous molecules by exterior elongation, compressionand flexing deformation is difficult so that a high amount of heat isgenerated to increase the temperature and thus to increase the physicalfatigue, but it is very small. On the contrary, the present yarn has ahigh degree of orientation in the amorphous portions which makes thepenetration of water and amines difficult, thereby decreasing thechemical deterioration to obtain excellent fatigue resistance.

Now, the process of the present invention will be described in detail.

The polyester used as the starting material may be a polyester with ahigh polymerization degree such that its intrinsic viscosity is at least0.85.

The intrinsic viscosity (η) may be calculated from the followingequation by determining the relative viscosity (ηr) of a solution of 8 gof sample in 100 ml of ortho-chlorophenol at 25° C. using an Ostwaldviscometer.

    η=0.0242 ηr+0.634

where ##EQU7## where t: dropping time of solution (in seconds)

to: dropping time of orthochlorophenol (in seconds)

d: density of solution (in g/cc)

do: density of orthochlorophenol (in g/cc)

The degree of polymerization is very important with respect to ultimatedimensional stability and fatigue resistance. In particular, a polymerwith a low molecular weight may be used advantageously for dimensionalstability but a high molecular weight is preferred for fatigueresistance. In the present invention, optimization of the whole of theproperties and a lowering of fatigue resistance can be achieved byselecting a polymer with an intrinsic viscosity of at last 0.85,preferably at least 1.0.

A high-speed spinning process is performed to obtain a highly orientedundrawn yarn with a birefringence of 0.03-0.08, preferably 0.05-0.08. Itis important to produce the undrawn yarn exhibiting unique shrinkagebehaviour in the prior step before producing the present yarn having athree-phase structure.

When the birefringence is less than 0.03, an excessive drawing isrequired in the drawing process to get a sufficient strength and modulusof elasticity for rubber-reinforcing fiber.

Accordingly, the degree of orientation is rapidly increased so that theexcessive drawing tension yields high residual stress leading to a highshrinkage of the fiber. On the contrary, when it is more than 0.08, theundrawn yarn is already in the state wherein the crystalline andamorphous portions exist in the mixed fashion so that the elongationphenomenon, which occurs when the oriented amorphous portions arecrystallized, does not occur and the lowering of strength by high-speedspinning cannot be avoided and thus the final dipped cord has poorproperties, especially a very low strength.

The birefringence of the undrawn yarn is proportional to the magnitudeof tension which the extruded yarn is subjected to upon reaching theglass transition temperature by cooling with quench air. The magnitudeof a tension depends upon the spinning speed, the discharge quantity peropening and the temperature of the quench air. In general, theorientation of the undrawn yarn is effected on the point that theextruded yarn from the spinneret reaches a temperature below the glasstransition temperature by cooling with quench air. In the presentinvention, the birefringence is controlled to be 0.03-0.08 with a hightension in the solidification point by heightening the spinning speed toincrease the speed of tensile deformation of the extruded yarn or byfixing the spinning speed and increasing the temperature of quench airor reducing the discharge quantity per opening. At this time, toheighten the tension in the solidification point, it is advantageousthat the bundle of filaments is slowly cooled so that the solidificationpoint moves as far as possible from the spinneret.

In the case that a high spinning speed is employed (2500-4000 m/min,preferably 3,000-3,600 m/min) to produce a highly oriented undrawn yarnas in the present process, the quench rate becomes different between theinner and outer layer of the polymer stream and thus the spinning stressalso becomes different, leading to production of a filamentary yarnhaving a skin-core structure wherein the inner and outer layer of thefilament have different structures from each other. To prevent formationof such a yarn, in the high-speed spinning process it is important tomelt-extrude the polymer through an orifice with a ratio of length todiameter (L/D) of 2-4 to form a multifilamentary yarn.

When L/D is less than 2, the temperature difference becomes severebetween the inner and outer layer of the filament, which results information of the skin-core structure, leading to poor properties. On thecontrary, when L/D exceeds 4, the pack pressure rapidly increases sothat the operation and handling become difficult and the pack period isto be reduced.

The lowering of strength owing to the temperature difference between theinner and outer layer of the filament can be deceased by increasing thequench temperature to a range of from 25° C. to the glass transitiontemperature of the polymer, desirably from 40° C. to 60° C. in order todecease the temperature difference between the inner and outer layer ofthe filament at the solidification point in a high-speed spinningprocess. When the temperature is less than 25° C., the filament may betoo quickly quenched and thus the tension at the solidification pointmay be decreased so that it may be difficult to obtain a highly orientedundrawn yarn. On the contrary, when exceeding Tg, the filaments may beinsufficiently quenched, so it may be impossible to process further.

Varying the discharge quantity per opening may have a great influenceupon the mechanical properties of the yarn. It is advantageous tomaintain the fineness of the yarn after drawing within 3 to 5 denier bycontrolling the spinning conditions and preventing a ununiform quench.

The present process is characterized by drawing at a low draw ratio andat a temperature below the crystallization temperature of the undrawnyarn.

Multi-step drawing of two or more step is preferably used in the presentprocess. The crystallization temperature of a highly oriented undrawnyarn produced by a high-speed spinning process is usually lower by morethan 10° C. than that of an undrawn yarn by a low-speed spinningprocess.

Thus, the drawing temperature is controlled to be in the range of fromthe glass transition temperature to 120° C., desirably 80° to 90° C.When the drawing temperature exceeds 120° C., fine crystals are alreadyformed before the orientation of the molecular chains and accordinglythe drawability is degraded. At temperatures below 80° C., the molecularchains lose their mobility, whereby the efficiency of drawing is low.

The total draw ratio is controlled to be in the range of about 1.4:1 to2.2:1, desirably 1.4:1 to 1.8:1 and 1.5:1 to 1.7:1. When this ratio isless than 1.4, the fiber attains insufficient strength and in the caseexceeding 2.2, high modulus values and low shrinkage cannot be achievedand the percentage of lowering of strength may be high.

The reason why multi-step drawing comprising two or more steps ispreferably used in the present process is as follows: if the drawing isperformed in one step by drawing to achieve about 70 percent of totaldraw ratio in the first drawing zone, the period of time taken is notenough for the tangled molecular chains to attain a fibrillar structureso that part of the molecular chains remain in the tangled state. Thiscauses a defect of structure and, accordingly, shrinkage by heat may beincreased.

According to the present invention, the shrinkage of the dipped cord canbe greatly decreased by use of a highly oriented undrawn yarn producedby the high-speed spinning process such that it is transformed into aliquid-like form rather than undergoes shrinkage when it is subjected toheat after drawing it under specific conditions.

In "Fundamental Aspects of Stress, Deformation and Phase Transitions inCrystallizable Polymers: Experiments with Poly (ethylene terephthalate)in Uniaxial Stress Fields" by P. Desai and A. S. Abhiraman in Journal ofPolymer Science: Part B, Vol. 26, 1988, p 1657-1675, it is reported thatas a result of experiments wherein the initially oriented amorphouspolymer was maintained at a temperature between the glass transitiontemperature and melting temperature and then its behaviour under stresswas observed, it is proved that shrinkage is originated from twistedmolecular chains in the oriented amorphous portion and transformation toliquid-like form by elongation occurs as the degree of orientationincreases when a stress higher than the shrinkage power is applied.

Therefore, the elongation and shrinkage behaviour upon application ofheat can be considered to be a phenomenon originated from the differenceof elongation power due to crystallization of the oriented amorphousmolecular chains.

Accordingly, in the present invention, the mechanism ofelongation-shrinkage behaviour is put to use so that the shrinkage canbe minimized.

The present inventors have found that in order to maximize theelongation behaviour like a liquid, crystallization by heat should notoccur during drawing. Accordingly, the drawing should be carried out ata low draw ratio and at a temperature below the crystallizationtemperature of the undrawn yarn. That is to say, when crystallization byheat already has occurred in the drawing process, because the orientedamorphous portion has been transformed into a crystalline portion, theelongation transformation occurring as the oriented amorphous portion ischanged to oriented crystals no longer occurs. The shrinkage behaviouronly occurs by disorientation of the amorphous molecular chains existingin the amorphous region, which leads to a high shrinkage value.

The present process is characterized in that the thermal treatment ofthe resulting drawn yarn is carried out at a temperature within thewhole range of 100° to 210° C. with effective intermediate temperatureincluding 120°, 140°, 160°, 180° and 200° C., for example.

When the temperature exceeds 210° C., the crystalline and amorphousportions may be previously defined in the yarn. Accordingly, the networkstructure characteristic of the present invention wherein thedevelopment of intermicrofibrillar tie molecules cannot be achieved andthe orientation of the crystalline portions is greatly increased and theorientation of the amorphous region is decreased. Therefore, thelowering of strength due to abnormal crystal growth in the subsequentdipping process can not be minimized.

The temperature is one of the important factors to determine thestructure of the yarn because in this thermal treatment a yarn withnearly completed orientation is treated. The temperature is required tobe in the range of 100° to 210° C., desirably 100° to 180° C. to producethe present polyester yarn for tire cord.

In general, the undrawn yarn before drawing will gain the characteristicproperties of the final yarn as it undergoes a drawing process whereinthe crystallization and orientation of molecular chains occur by theheat employed in drawing. The orientation in drawing occurs concurrentlyin the crystalline and amorphous portions, and drawing tension of theamorphous portion is higher than the crystalline portions. Thus, when ayarn for tire cord having such a microstructure is subjected to twistingor to a dipping process to be formed into a tire cord, the mechanicalproperties of the yarn are often seriously degraded.

In the present invention, this problem is solved by subjecting, the yarnafter the drawing process to a relaxing treatment at a relax ratio of 3to 6 percent at a temperature below 140° C., which is the temperature atwhich the amorphous molecular chains initiate mobilization, that is, theloss tangent (tan δ) is at a maximum.

When the relaxing temperature is greater than 140° C., the yarn willinitiate the creation of defects in the crystalline structure ordestruction of the same upon the application of heat in the subsequentdipping process.

When the relax ratio is less than 3 percent, the wind-up tension may beincreased and the removal of residual drawing stress may beinsufficient, thereby making process difficult. When the relax ratio ismore than 6 percent, the strength efficiency may be deceased and theresulting lowering of shrinkage may be so small that the effect cannotremain in the dipped cord.

The resulting polyester filamentary yarn of the present inventionproduced by the above process is then subjected to a cord conversionprocess to produce a tire cord of the present invention.

The cord conversion process is described in detail hereinafter. Thepresent yarn is subjected in sequence to dipping in a rubber solution,drying, thermal treatment and normalizing thereby to produce a tirecord. In the above thermal treatment a tension in the range of 0.2 to0.6 g/d, a temperature in the range of 220° to 250° C. is suitablyemployed.

If the tension exceeds 0.6 g/d or the temperature exceeds 250° C., astress much higher than the elongation power originated from thecrystallization of the oriented amorphous molecular chains may beapplied against the yarn and then it may remain as a residual stressfinally in the dipped cord, leading to an increase in shrinkage. Whenthe tension is less than 0.2 g/d, the shrinkage may be decreased but thestrength is lowered due to the undesirable growth of the amorphousmolecular chains owing to disorientation and folding of the chains. Andif the above temperature is less than 220° C., the adhesion of therubber solution may be insufficient, the shrinkage may increase and,further, it may be impossible to obtain a tire cord with a highcrystallinity.

Generally, filaments of in total 1,000 denier, are twisted in more thantwo strands and formed into a fabric, then this fabric is dipped into aconventional rubber solution and dried. Subsequently, the fabric isthermally treated at the above described temperature and tension, andnormalized to obtain a cord fabric, from which the dipped cord of thepresent invention is obtained.

The thus obtained dipped cord exhibits a strength at 10% elongation ofat least 100N measured by the use of an instron type tensile strengthtester, and a shrinkage (S) of at most 3.5 percent after thermallytreating it in a dry-heat oven at 177° C. under a dead weight loading of20 g during 2 minutes. In addition, the tire cord after measuring theshrinkage (S) exhibits a high value of strength at 10% elongation (L) ofat least 65N.

According to the present invention, a tire cord with a coefficient ofdimensional stability (L/S) of at least 20 obtained by multiplying thevalue (L) by a reciprocal of the shrinkage (S) can be produced.

Now, the present invention will be described in more detail by theExamples and Comparative examples which are illustrative only and arenot intended to limit the scope of the invention.

EXAMPLE 1 TO 20 AND COMPARATIVE EXAMPLE 1 TO 10

A polyethylene terephthalate polymer having an intrinsic viscosity of1.0 to 1.1 and a terminal carboxyl group content of about 15 eq/10⁶ gwas used as a starting material. The polymer was melt-spun at 305° C.

In the melt spinning, a spinneret containing .[.192.]. .Iadd.250.Iaddend.holes and a diameter of 0.5 mm was used in the extrusion. TheL/D (ratio of length to diameter) of the orifice was varied within therange of from 2 to 4. A shroud with 200 mm length maintained at 330° C.was placed directly under the spinneret and below the shroud quenchingand solidification was carried out with quench air at a temperature lessthan 80° C.

The other process conditions employed to produce the polyesterfilamentary yarns are described in Table 1 and 2 which follow.

The flow rate was controlled to produce a final polyester yarn with afineness of 1,000 total denier. The properties of the obtained yarns aredescribed also in Table 1 and 2 which follow.

EXAMPLE 21 TO 28 AND COMPARATIVE EXAMPLE 11 TO 20

The drawn filamentary yarn produced in the above Examples andComparative examples was subjected to twisting consisting of first twistof 49 times/10 cm in Z direction and second twists of 49 times/10 cm isS direction and two times of doubling and then formed into a fabric. Theresulting fabric was dipped in a rubber treatment solution comprisingresorcinol formalin latex and epoxy isocianate and then dried at 160° C.during 60 seconds.

Thereafter, the dipped fabric was thermally treated under the conditionsdescribed in Table 3 and 4 which follow, relaxed at 1.5 percent, andnormalized at 245° C. for 60 seconds, to finally obtain a polyester tirecord.

The properties of the so obtained tire cord are also described in Table3 and 4 which follow.

The tests of the properties appearing in Table 1 to 4 were performedaccording to the following methods.

1. tenacity and elongation: in accordance with JIS-L1017 method.

Instrument: low-speed elongation type tensile strength tester fromInstron Co., Ltd., tensile speed: 300 mm/min, length of sample: 250 mm,atmospheric conditions: 25° C., 65% RH.

2. medium elongation

The elongation value at a load of 4.5 g/d in the elongation load curveobtained in accordance with JIS-L1017 method using an instrument same asthat used in above 1).

3. shrinkage of yarn:

The value (Δδ, in percent) calculated from the following equationwherein L₀ is the length of a sample measured under a load of 20 g afterit has been placed at 25° C., 65% RH during more than 24 hours, and L₁is the length after it has been placed in the oven at 150° C. during 3minutes under zero load. ##EQU8##

4. shrinkage of cord

The value calculated from the following equation wherein l₀ is thelength of a cord sample taken from a cord fabric measured under a deadweight loading of 20 g after it has been placed at 25° C., 65% RH duringmore than 24 hours, and 11 is the length of same measured after it hasbeen placed in an oven at 177° C. during minutes under a dead weightloading of 20 g. ##EQU9##

5. Percentage of retained strength of cord: in accordance with ASTM D885 method.

The value obtained from the following formula by measuring the strengthof a cord sample taken from a tire, before and after hours rotation atan inner tube pressure of 3.5 kg/cm², rotation speed of 850 rpm and tubeangle of 80° C. ##EQU10##

                                      TABLE 1                                     __________________________________________________________________________    Parameters   Example No.                                                      and          Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example             properties   1    2    3    4    5    6    7    8    9    10                  __________________________________________________________________________    I.V. of polymer                                                                            1.1  1.0  1.0  1.0  1.0  1.0  1.1  1.0  1.0  1.0                 L/D of orifice                                                                             2    2    2    3    3    3    3    3    3    3                   quench air temp. (° C.)                                                             25   40   25   60   50   40   40   40   50   60                  spinning speed (m/min)                                                                     2500 3500 3800 3400 2500 3600 3000 3200 4000 2700                birefringence                                                                              0.032                                                                              0.060                                                                              0.066                                                                              0.069                                                                              0.040                                                                              0.070                                                                              0.060                                                                              0.064                                                                              0.080                                                                              0.044               temp. of 1st draw zone                                                                     90   90   100  70   80   80   90   80   90   90                  (° C.)                                                                 draw ratio of 1st draw                                                                     1.80 1.64 1.65 1.65 1.85 1.76 1.70 1.65 1.60 1.80                temp. of 2nd draw zone                                                                     100  90   100  90   110  90   90   90   90   100                 (° C.)                                                                 draw ratio of 2nd draw                                                                     1.2  1.1  1.06 1.1  1.11 1.05 1.06 1.06 1.03 1.06                total draw ratio                                                                           2.08 1.75 1.60 1.76 1.97 1.79 1.74 1.68 1.58 1.84                temp. of heat                                                                              200  180  190  180  160  170  170  180  170  180                 treatment (° C.)                                                       relax temp. (° C.)                                                                  30   100  80   80   30   140  130  120  130  30                  relax ratio (%)                                                                            3.5  3.2  3.0  3.0  4.0  3.0  3.6  4.0  4.0  3.5                 tenacity (g/d)                                                                             8.5  8.0  7.0  7.3  8.3  8.6  7.4  7.3  7.2  7.6                 elongation (%)                                                                             12.0 12.6 15.2 13.2 12.2 12.1 13.5 14.2 15.0 14.0                medium elongation (%)                                                                      4.5  5.0  7.2  7.0  5.2  4.4  6.2  6.3  7.2  6.2                 shrinkage (%)                                                                              12.0 11.0 10.3 11.2 13.0 12.6 11.4 11.2 10.0 11.3                maximum thermal stress                                                                     0.48 0.46 0.29 0.43 0.50 0.50 0.40 0.38 0.28 0.34                (g/d, 60-250° C.)                                                      thermal stress                                                                             decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease            behavior beyond 210° C.                                                long period (Å)                                                                        154  150  153  140  148  155  140  150  139  135                 crystallite size of                                                                        65   64   65   60   62   63   58   64   64   63                  (lo5) plane (Å)                                                           crystal volume                                                                             1.45 1.43 1.42 1.40 0.92 1.32 0.54 1.30 0.62 0.54                (× 10.sup.5 Å.sup.3)                                                crystalline  0.924                                                                              0.921                                                                              0.911                                                                              0.901                                                                              0.924                                                                              0.938                                                                              0.920                                                                              0.911                                                                              0.912                                                                              0.919               orientation function                                                          (fc)                                                                          amorphous orientation                                                                      0.645                                                                              0.634                                                                              0.622                                                                              0.621                                                                              0.632                                                                              0.652                                                                              0.610                                                                              0.611                                                                              0.600                                                                              0.620               function (fa)                                                                 terminal modulus (g/d)                                                                     19.5 18.5 16.4 16.9 18.9 19.9 16.6 17.0 16.5 17.1                crystallinity (wt %)                                                                       45   42   43   40   38   35   42   43   41   40                  fa (1 - Xc)  0.355                                                                              0.368                                                                              0.355                                                                              0.373                                                                              0.392                                                                              0.424                                                                              0.354                                                                              0.348                                                                              0.354                                                                              0.372                            7.8  11.0 10.5 12.6 13.3 13.9 11.2 10.7 11.9 12.2                __________________________________________________________________________    Parameters   Example No.                                                      and          Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example             properties   11   12   13   14   15   16   17   18   19   20                  __________________________________________________________________________    I.V. of polymer                                                                            1.1  1.0  1.1  1.1  1.1  1.1  1.1  1.0  1.1  1.1                 L/D of orifice                                                                             2    2    2    3    2    3    3    4    4    4                   quench air temp. (° C.)                                                             25   25   40   40   60   60   50   60   30   60                  spinning speed (m/min)                                                                     2700 2900 3400 3800 3000 2900 3500 3200 4000 3600                birefringence                                                                              0.042                                                                              0.046                                                                              0.065                                                                              0.075                                                                              0.057                                                                              0.052                                                                              0.068                                                                              0.065                                                                              0.080                                                                              0.072               temp. of 1st draw zone                                                                     90   90   90   80   80   80   80   80   100  70                  (° C.)                                                                 draw ratio of 1st draw                                                                     1.90 2.05 1.70 1.50 135  1.50 1.65 1.60 1.40 1.70                temp. of 2nd draw zone                                                                     100  100  90   90   90   120  90   80   100  90                  (° C.)                                                                 draw ratio of 2nd draw                                                                     1.14 1.05 1.03 1.05 1.32 1.33 1.04 1.07 1.14 1.08                total draw ratio                                                                           2.10 2.07 1.67 1.53 1.71 1.93 1.63 1.63 1.53 1.76                temp. of heat                                                                              120  160  180  180  160  170  180  180  160  170                 treatment (° C.)                                                       relax temp. (° C.)                                                                  30   30   120  120  100  100  30   30   140  140                 relax ratio (%)                                                                            3    4    4.5  3    4    3.5  5    5    4    4                   tenacity (g/d)                                                                             7.7  8.4  7.6  7.2  7.7  8.0  7.5  7.4  7.0  8.0                 elongation (%)                                                                             12.8 13.0 14.8 15.1 14.9 12.5 14.2 14.1 15.4 11.3                medium elongation (%)                                                                      5.4  4.9  6.8  7.2  6.7  5.2  7.0  6.5  7.2  4.6                 shrinkage (%)                                                                              13.0 12.4 11.4 10.2 11.2 12.5 11.2 11.2 10.4 12.6                maximum thermal stress                                                                     0.48 0.42 0.36 0.24 0.36 0.43 0.40 0.38 0.36 0.46                (g/d, 60-250° C.)                                                      thermal stress                                                                             decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease                                                                           decrease            behavior beyond 210° C.                                                long period (Å)                                                                        153  152  150  151  140  154  142  152  153  152                 crystallite size of                                                                        61   65   63   64   60   63   60   60   63   64                  (lo5) plane (Å)                                                           crystal volume                                                                             1.51 1.49 1.43 1.44 1.32 1.52 1.43 1.40 1.45 1.48                (× 10.sup.5 Å.sup.3)                                                crystalline  0.924                                                                              0.935                                                                              0.929                                                                              0.911                                                                              0.928                                                                              0.938                                                                              0.919                                                                              0.921                                                                              0.909                                                                              0.936               orientation function                                                          (fc)                                                                          amorphous orientation                                                                      0.639                                                                              0.632                                                                              0.621                                                                              0.601                                                                              0.620                                                                              0.609                                                                              0.611                                                                              0.609                                                                              0.601                                                                              0.650               function (fa)                                                                 terminal modulus (g/d)                                                                     19.6 19.4 18.0 17.5 18.0 18.8 18.6 18.2 17.0 19.7                crystallinity (wt %)                                                                       32   38   42   40   35   36   41   44   39   33                  fa (1 - Xc)  0.445                                                                              0.392                                                                              0.360                                                                              0.361                                                                              0.403                                                                              0.390                                                                              0.360                                                                              0.341                                                                              0.367                                                                              0.436                ##STR1##    14.9 13.7 11.1 12.5 14.2 13.8 11.8 10.2 12.4 14.8                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                 comp. example No.                                                Parameters   comp.                                                                              comp.                                                                              comp.                                                                              comp.                                                                              comp.                                                                              comp.                                                                              comp.                                                                              comp.                                                                              comp.                                                                              comp.               and          example                                                                            example                                                                            example                                                                            example                                                                            example                                                                            example                                                                            example                                                                            example                                                                            example                                                                            example             properties   1    2    3    4    5    6    7    8    9    10                  __________________________________________________________________________    I.V. of polymer                                                                            1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0  1.0                 L/D of orifice                                                                             2.5  2.5  2.5  3    3    3    4.5  3    3    3                   quench air temp. (° C.)                                                             25   25   25   25   40   40   40   25   40   40                  spinning speed (m/min)                                                                     1800 1800 2500 3000 3000 4800 Handling                                                                           600  3050 3500                birefringence                                                                              0.020                                                                              0.020                                                                              0.03 0.036                                                                              0.040                                                                              0.098                                                                              was  0.003                                                                              0.04 0.052               temp. of 1st draw zone                                                                     90   90   90   130  100  80   impossible                                                                         110  110  110                 (° C.)                                                                 draw ratio of 1st draw                                                                     2.15 2.15 2.10 1.80 1.80 1.80      4.0  1.5  1.3                 temp. of 2nd draw zone                                                                     100  100  100  150  130  90        220  220  220                 (° C.)                                                                 draw ratio of 2nd draw                                                                     1.07 1.07 1.05 1.03 1.03 1.04      1.4  1.1  1.03                total draw ratio                                                                           2.22 2.22 2.17 1.78 1.72 1.28      5.26 1.58 1.30                temp. of heat                                                                              190  245  220  200  220  180       240  240  240                 treatment (° C.)                                                       relax temp. (° C.)                                                                  150  140  230  130  240  140       180  180  180                 relax ratio (%)                                                                            3.5  3.5  4    4    7    5         6    4    3                   tenacity (g/d)                                                                             8.9  8.8  8.5  Handling                                                                           8.0  6.2  (Pack                                                                              8.9  8.14 7.95                elongation (%)                                                                             11.0 12.1 13.4 was  14.9 15.8 pressure                                                                           14.1 13.7 14.4                medium elongation (%)                                                                      4.0  4.4  5.0  impossible                                                                         6.5  8.4  was  4.8  6.6  6.5                 shrinkage (%)                                                                              11.3 7.4  7.0       5.0  5.2  increased)                                                                         10.5 5.9  5.6                 maximum thermal stress                                                                     0.56 0.49 0.42      0.30 0.24      0.65 0.54 0.51                (g/d, 60-250° C.)                                                      thermal stress                                                                             increase                                                                           increase                                                                           increase  increase                                                                           increase  increase                                                                           increase                                                                           increase            behavior beyond 210° C.                                                long period (Å)                                                                        153  157  159       161  156       155  142  142                 crystallite size of                                                                        64   66   67        69   68        68   65   65                  (lo5) plane (Å)                                                           crystal volume                                                                             1.50 1.55 1.57      1.58 1.54      2.0  1.57 1.55                (× 10.sup.5 Å.sup.3)                                                crystalline  0.946                                                                              0.932                                                                              0.941     0.943                                                                              0.911     0.932                                                                              0.944                                                                              0.945               orientation function                                                          (fc)                                                                          amorphous orientation                                                                      0.620                                                                              0.502                                                                              0.501     0.468                                                                              0.511     0.435                                                                              0.500                                                                              0.510               function (fa)                                                                 terminal modulus (g/d)                                                                     23.1 19.6 18.4      16.4 18.4      34.1 16   14                  crystallinity (wt %)                                                                       47   55   51        54   48        50   53   53                  fa (1 - Xc)  0.329                                                                              0.226                                                                              0.245     0.215                                                                              0.266     0.217                                                                              0.235                                                                              0.240                ##STR2##    4.8  2.9  3.6       2.7  4.5       3.9  3.4  3.2                 __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________               Example No.                                                                   Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                    21   22   23   24   25   26   27   28                              __________________________________________________________________________    example no. of the                                                                       1    4    8    9    12   15   18   19                              used yarn                                                                     temperature (° C.)                                                                240  245  230  240  220  240  250  240                             tension (g/d)                                                                            0.6  0.3  0.5  0.3  0.6  0.3  0.2  0.5                             strength (L10; Newton)                                                                   104  106  102  105  107  106  102  101                             shrinkage (S; %)                                                                         3.1  3.1  2.9  3.0  3.3  3.2  3.0  3.2                             strength after dry-heat                                                                  72.1 72.8 71.5 72.5 74.2 73.0 70.9 70.0                            (L; Newton)                                                                   L/S        23.2 23.4 24.6 24.1 22.5 22.8 23.6 21.9                            maximum thermal stress                                                                   0.07 0.08 0.06 0.06 0.08 0.09 0.07 0.09                            (g/d)                                                                         percentage of retained                                                                   96   96   98   97   95   95   97   94                              strength of cord (%)                                                          __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________               comp. example No.                                                             comp.                                                                             comp.                                                                             comp.                                                                             comp.                                                                              comp.                                                                             comp.                                                                             comp.                                                                             comp.                                                                             comp.                                                                             comp.                                    example                                                                           example                                                                           example                                                                           example                                                                            example                                                                           example                                                                           example                                                                           example                                                                           example                                                                           example                                  11  12  13  14   15  16  17  18  19  20                            __________________________________________________________________________    example no. of the                                                                       1   8   15  19   comp.                                                                             comp.                                                                             comp.                                                                             comp.                                                                             comp.                                                                             comp.                         used yarn                   example                                                                           example                                                                           example                                                                           example                                                                           example                                                                           example                                                   1   3   5   8   9   10                            temperature (° C.)                                                                210 240 200 250  240 245 230 240 250 240                           tension (g/d)                                                                            0.4 0.1 0.1 0.7  0.6 0.3 0.5 0.3 0.2 0.5                           strength (L10; Newton)                                                                   104 95  96  The cord                                                                           82  79  83  82  80  80                                                   was cut                                                shrinkage (S; %)                                                                         4.1 3.5 3.4      4.0 3.9 4.9 4.0 3.5 4.7                           strength after dry-heat                                                                  70.2                                                                              65.1                                                                              66.2     50.4                                                                              52  56  54.2                                                                              53.5                                                                              52                            (L; Newton)                                                                   L/S        17.1                                                                              18.6                                                                              19.4     12.6                                                                              13.3                                                                              11.4                                                                              13.6                                                                              15.3                                                                              11.1                          maximum thermal stress                                                                   0.18                                                                              0.09                                                                              0.08     0.25                                                                              0.22                                                                              0.29                                                                              0.19                                                                              0.15                                                                              0.20                          (g/d)                                                                         percentage of retained                                                                   90  93  92       85  88  90  87  86  87                            strength of cord (%)                                                          __________________________________________________________________________

From the result in Table 3, it can be seen that the present tire cordhas a shrinkage less than 3.5 percent at 150° C. over 30 minutes underzero tension and a coefficient of dimensional stability of at least 20and thus exhibits excellent dimensional stability. Also, it can be seenthat the present tire cord has a strength at 10% elongation of at least100N and a strength at 10% elongation after the dry-heat treatment of atleast 65N accordingly it has good fatigue resistance. Though the presentinvention has been described by the preferred embodiments, it isunderstood that the variation and modification apparent to the skilledin this art is possible. Such variation and modification should beconsidered to be included within the spirit and scope of the presentclaims which follow.

What is claimed is:
 1. A polyester filamentary yarn comprising at least90 mol. % polyethylene terephalate and having a fineness of 3-5 denierper filament wherein said polyester has a crystalline orientationfunction (fc) of at most 0.94, percent crystallinity (Xc) of 0.320-0.45,and has a three-phase microstructure consisting of crystalline,amorphous and mesomorphous portions, the proportion of the mesomorphousportion is 5-15 percent based upon the total amount of crystalline,amorphous and mesomorphous portions of said polyester, and wherein theamount of the mesomorphous portion and the total amount of crystalline,amorphous and mesomorphous portions are determined by the equation

    percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyestercalculated from the X-ray intensity of the peak for the (010) crystalplane in a quantitative equatorial diffraction pattern of the polyesterand I total is the sum of the crystalline, mesomorphous and amorphousportions of the polyester calculated from said quantitative equatorialX-ray diffraction pattern.
 2. The polyester yarn of claim 1, whereinsaid yarn has:i) a crystalline orientation function (fc) of at most0.94, ii) an amorphous orientation function (fa) of at least 0.60, iii)a long period value of at most 155 Å, where fa(1-Xc)>0,330 where Xc isthe percent crystallinity and is a value of 0.30-0.45.
 3. The polyesteryarn of claim 2, wherein said polyester has a three-phase microstructureconsisting of crystalline, amorphous and mesomorphous portions, theproportion of the mesomorphous portion is 5-15 percent based upon thetotal amount of crystalline, amorphous and mesomorphous portions of saidpolyester, and wherein the amount of the mesomorphous portion and thetotal amount of crystalline, amorphous and mesomorphous portions aredetermined by the equation

    percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyestercalculated from the X-ray intensity of the peak for the (010) crystalplane in a quantitative equatorial diffraction pattern of the polyesterand I total is the sum of the crystalline, mesomorphous and amorphousportions of the polyester calculated from said quantitative equatorialX-ray diffraction pattern.
 4. The polyester yarn of claim 2, whereinsaid yarn has a terminal modulus of at most 20 g/d.
 5. The polyesteryarn of claim 1, wherein said yarn has a terminal modulus of at most 20g/d.
 6. The polyester yarn of claim 1, where said yarn has a crystalvolume of 0.5-1.54×10⁵ Å³.
 7. A polyester filamentary yarn comprising atleast 90 mol % polyethylene terephthalate and having a fineness of 3-5denier per filament, wherein said yarn is:i) a crystallinity of 30-45percent by weight, ii) a crystallite size in the of at most 65 Å iii) acrystal volume of 0.5×10⁵ Å³ -1.54×10⁵ Å³.
 8. The polyester yarn ofclaim 7, wherein said yarn has:i) a crystalline orientation function(fc) of at most 0.94, ii) an amorphous orientation function (fa) of atleast 0.60, iii) a long period value of at most 155 Å, where fa(1-Xc)0.330 where Xc is the percent crystallinity and is a value of 0.30-0.45.9. The polyester yarn of claim 8, wherein said polyester has athree-phase microstructure consisting of crystalline, amorphous andmesomorphous portions, the proportion of the mesomorphous portion is5-15 percent based upon the total amount of crystalline, amorphous andmesomorphous portions of said polyester, and wherein the amount of themesomorphous portion and the total amount of crystalline, amorphous andmesomorphous portions are determined by the equation

    percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyestercalculated from the X-ray intensity of the peak for the (010) crystalplane in a quantitative equatorial diffraction pattern of the polyesterand I total is the sum of the crystalline, mesomorphous and amorphousportions of the polyester calculated from said quantitative equatorialX-ray diffraction pattern.
 10. The polyester yarn of claim 8, whereinsaid yarn has a terminal modulus of at most 20 g/d.
 11. A polyesterfilamentary yarn comprising at least 90 mol % polyethylene terephthalateand having a fineness of 3-5 denier per filament, wherein said yarn hasa maximum thermal stress of at most 0.5 g/d in the temperature range offrom 60° C. to 250° C. and exhibits decreasing slope above 210° C. in athermal stress versus temperature curve.
 12. The polyester yarn of claim11, wherein said yarn has:i) a crystalline orientation function (fc) ofat most 0.94, ii) an amorphous orientation function (fa) of at least0.60, iii) a long period value of at most 155 Å, where fa(1-Xc)>0.330where Xc is the percent crystallinity and is a value of 0.30-0.45. 13.The polyester yarn of claim 12, wherein said polyester has a three-phasemicrostructure consisting of crystalline, amorphous and mesomorphousportions, the proportion of the mesomorphous portion is 5-15 percentbased upon the total amount of crystalline, amorphous and mesomorphousportions of said polyester, and wherein the amount of the mesomorphousportion and the total amount of crystalline, amorphous and mesomorphousportions are determined by the equation

    percent of mesomorphous portion=I meso/I total×100,

wherein I meso is the amount of mesomorphous portion in the polyestercalculated from the X-ray intensity of the peak for the (010) crystalplane in a quantitative equatorial diffraction pattern of the polyesterand I total is the sum of the crystalline, mesomorphous and amorphousportions of the polyester calculated from said quantitative equatorialX-ray diffraction pattern.
 14. The polyester yarn of claim 12, whereinsaid yarn has a terminal modulus of at most 20 g/d.
 15. A polyesterfilamentary yarn comprising at least 90 mol % polyethylene terephthalateand having a fineness of 3-5 denier per filament, wherein said yarnhas:i) a crystalline orientation function (fc) of at most 0.94, ii) anamorphous orientation function (fa) of at least 0.60, ii) a long periodvalue of at most 155 Åand where said yarn is characterized in thatfa(1-xc)>0.330 where Xc is the percent crystallinity and is a value of0.30-0.45.